A common characteristic of coverings, coatings or other treatments used in reducing or otherwise modifying the reflection-transmission characteristics of surfaces is that these approaches are all passive.
In this regard, such techniques rely on the use of materials which have the appropriate mechanical properties and which are geometrically configured to produce the performance or result desired. To cite a simple example, materials that provide high sound absorption can be used to absorb incoming sound and thus reduce reflections. Similarly, special geometric configurations have been provided which spatially diffract or diffuse the reflected energy. In other applications, highly reflective surface coatings, such as decoupling coatings, are used to prevent sound from crossing a surface.
All such broad band passive coatings or coverings require that the coating or covering material be of a reasonably substantial thickness. Typically the coating thickness must be greater than one-fifth of the wavelength of the lowest frequency of the sound involved. At frequencies greater than about 10 kHz in water (and about 2 kHz in air), this thickness limitation is not a serious consideration because the corresponding material thickness is less than a few centimeters. However, at much lower frequencies, the thickness required can be prohibitive in many applications (e.g., a thickness of 30 cm is required at 1 Khz in water and 200 Hz in air).
Prior art coatings and other devices for absorbing or otherwise modifying acoustic waves include those disclosed in U.S. Pat. Nos. 4,883,143 (Lagier); 4,390,976 (Eynck); 2,000,806 (White); 3,515,910 (Fritz); 4,828,932 (Morimoto et al.); 4,628,490 (Kramer et al.); 4,152,474 (Cook, deceased et al.); and 4,346,782 (Bohm). Briefly considering these patents, the Lagier patent discloses an anechoic coating for preventing the reflection of acoustic waves which includes a first elastic layer of low compressibility and high absorbency and a second layer of high compressibility. A set of plates covers the second layer and rods fixed to the plates transmit vibrations to the first layer. The Eynck patent discloses an underwater acoustic signal conditioning device including a skirt baffle positioned adjacent to a hull surface, an acoustic conditioning module comprising inner and outer spaced cover plates and "tuned" damping elements secured thereto, and an outer layer containing a plurality of hydroplanes. The White patent discloses an apparatus for sound modification wherein sound waves are split into various components and the components are recombined in a different phase relationship. The Fritz et al. patent discloses an acoustic energy absorbing material for absorbing sound energy under water wherein particles of piezoelectric or ferroelectric material convert incident sound wave energy into electrical energy that is dissipated in a conductive coating. The Morimoto et al. patent discloses a porous sound absorbing metallic material comprising a laminate of expanded metal and metal fiber. The Kramer et al. patent discloses a wideband sonar energy absorber comprising a non-conductive elastometer matrix having piezoelectric or magnetstrictive particles disposed therein. The Cook patent discloses an acoustic absorber comprising an organic polymer coating on a substrate which covers the substrate and partially fills openings therein. The Bohm patent discloses a sound absorbing coating comprising two layers of viscoelastic material wherein the modulus of elasticity of the outer layer is substantially greater than that of the inner layer.
These passive coatings all hare the common defect that, to be effective, they must be thick compared to expected acoustic wavelengths. This places a practical limit on their usefulness at lower frequencies.